For using the Philips 80C51MX, you need to have an PK51 package.
Details about the Philips 80C51MX are discussed in the Application
Note 160 available at www.keil.com.

C Program Example for the Philips 80C51MX Architecture that shows how
to define a huge memory pool with 19000 elements 20 byte each.

You may adapt the file RECORD.H to configure the pool size for your needs.